Chlorella is a green, single-celled protoctist that is photosynthetic. Some students immobilised cells from a culture of this protoctist using the same method as used for immobilising enzymes. The students used the respirometer shown in Fig. 1.1 to measure oxygen uptake by the immobilised Chlorella cells. [Figure 1.1]

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

1. [3] idea of: mix (equal volumes of), Chlorella / suspension, and sodium alginate ; (slowly) drip mixture into calcium chloride (solution); using a, syringe / pipette;
2. [2] independent variable: temperature; dependent variable: distance travelled by the coloured liquid ;
3. [2] any 2 from: idea that 1 (suitable because) these are temperatures at which, enzymes are expected to be functioning / Chlorella may survive ; 2 (suitable because) gives enough range for at least five temperatures ; 3 (not suitable because) does not show effect of very, high / low, temperatures ;
4. [7] allow in context of using the same respirometer for each temperature, or separate respirometers set up in the same way for each temperature any 7 from: 1 idea of adding a fixed volume of water to respirometer ; 2 idea of adding, fixed number / same mass, of immobilised algal balls (to the water below the shelf with CO2 absorbent); 3 idea of attaching the tap, capillary tubing and scale so that they are airtight; 4 idea of opening the tap (using syringe) to add air ; 5 idea of dipping capillary tube into liquid / setting position of liquid (using syringe); 6 idea of covering respirometer tube (containing Chlorella) with, metal foil / light proof material; A carry out in a dark room 7 idea of placing respirometer tube into a water-bath at a known temperature ; 8 idea of opening tap and leaving to equilibrate ; 9 idea of closing the tap and marking position of liquid; 10 idea of measuring movement of liquid along the scale for 5 minutes ; 11 idea of doing minimum of three measurements (in sequence) and taking a mean; 12 idea of repeating experiment at each temperature, e.g. 20 °C, 30 °C, 40 °C and 50 °C ; A temperatures within the range of 10°C to 50°C 13 low risk experiment or medium risk experiment due to (named) CO2 absorbent harmful and wear gloves ; 14 AVP;
5. [2] idea of: find radius of tubing in mm and use πr² (to find area); multiply by distance moved in mm ; A in terms of a formula
6. [2] any 2 from: 1 idea that oxygen uptake increases as temperature increases and because enzymes, gain more kinetic energy / collide more frequently / form more enzyme-substrate complexes ; 2 idea that increases faster up to 30°C and then slows down and because, optimum temperature exceeded / other factors become limiting; A doubles every 10°C increase up to 30°C then slows / Q10 plus explanation 3 AVP; e.g. idea that increase in metabolic rate is due to increasing enzyme activity